Diarrheal diseases caused by infectious agents continue to pose a serious health problem, especially in infants. It is, therefore, necessary to develop novel efficacious drugs to treat diarrhea. Intestinal serotonin transporter (SERT), involved in regulation of luminal 5-HT availability, is an important candidate to target because of its implications in several diarrheal disorders. The proposed studies are focused at examining the role of serotonin transporter (SERT) in the pathophysiology of diarrhea associated with infection by an important food-borne pathogen, Enteropathogenic E. coli (EPEC). Because of the lack of a known toxin, pathophysiology of EPEC-induced early diarrhea remained elusive for many years. Emerging studies have revealed that mechanism(s) underlying EPEC associated diarrhea are multi-factorial. Our recent studies showed that EPEC inhibits SERT in the intestinal epithelial cells via activation of protein tyrosine phosphatases. However, the detailed mechanisms underlying EPEC induced inhibition of SERT are not known. We hypothesize that EPEC induced signaling events alter SERT phosphorylation levels to cause its inhibition that may contribute to associated diarrhea. The present studies will systematically examine the effects of EPEC infection on SERT utilizing both in vitro and in vivo models. Studies proposed in Specific Aim 1 will identify the specific serine/threonine/tyrosine phosphatases involved in SERT modulation post-infection and will identify the structural domains that harbor SERT phosphorylation site(s). Specific Aim 2 will critically elucidate the mechanisms of acute inhibitio of SERT in response to EPEC infection by assessing the role of cellular phosphatases in the native mouse ileum. In addition, the effect of phosphatases on serotonergic machinery (5-HT synthesis and release) will be examined to model the net impact of EPEC infection in complex native intestine. Overall, the proposed studies will help provide valuable insights into the host-microbial epithelial interaction underlying pathophysiology of EPEC- associated diarrhea as well as will increase our understanding of phosphorylation/dephosphorylation mechanisms governing SERT activity under physiological and pathophysiological conditions.